Cardiac pacemakers known as "double chamber" pacemakers have a function, among others, to follow the spontaneous cardiac rhythm detected at the level of the atrium (referred to as the "atrial rhythm" or the "atrial frequency"), and to stimulate the ventricle in synchronism with the detected atrial rhythm. Nevertheless, this synchronization presents an upper rate limit known as the "maximal frequency of ventricular stimulation". If the patient develops a crisis of atrial tachycardia (also called a supraventricular tachycardia or "SVT"), then it is necessary to limit the ventricular synchronization to a controlled or defined range. This is to avoid stimulating the ventricle too rapidly, beyond a maximal frequency acceptable to the individual.
Several solutions have been implemented to control the ventricular stimulation frequency in the case of such a detected excessive acceleration of the atrial frequency. All these devices have for an object to try to keep a certain synchronization beyond the maximal frequency and, if the atrial frequency continues to exceed the maximal ventricular synchronization frequency, they begin to operate in a mode of asynchronous stimulation, tending to reduce the ventricular stimulation frequency to a lower and more tolerable ventricular frequency. Then, as soon as the atrial frequency falls below the maximal synchronization limit, a phase of progressive re-synchronization is started.
FR-A-2 544 989 refers to a process of ventricular synchronization for a sensed atrial frequency that is above the maximal ventricular synchronization frequency, by lengthening the atrial-ventricular delay (A-V delay) for a first atrial frequency range, so as not to sense (i.e., to skip) a selected atrial signal among a sequence of atrial signals, in the elevated frequency range. If these conditions persist, then the ventricular stimulation is desynchronized from the sensed atrial frequency, and decreased until a base frequency is reached. This mode of functioning is called "fallback".
U.S. Pat. No. 4,932,046 follows in many respects the same idea. It differs, however, in that the desynchronization occurs immediately, i.e., as soon as the atrial frequency goes above the maximal ventricular synchronization frequency, and the ventricular frequency then changes to a frequency which is indicated by a physiological sensor.
EP-A-0 448 193 presents yet a different approach of the problem. The information delivered by the physiological sensor serves to determine the maximal ventricular synchronization frequency. The desynchronization of the ventricular stimulation from the atrial frequency is to a frequency corresponding to that indicated by the sensor. The desynchronization is realized in a similar manner as described in the preceding cases.
In all these proposals of the prior art, the information of the physiological sensor is, however, used only after desynchronization, for the adjustment of the ventricular frequency in connection with the fallback phase. In no case, however, is the information provided by the sensor is used to establish and confirm the onset of a pathological acceleration of the atrium.